The invention relates to a pump/nozzle of the type in which the pump piston is brought to a halt by means of the fuel enclosed in the filling chamber after the pressure line leading to the injection valve is closed, this closing operation thus determining the end of fuel delivery. The enclosed fuel thereby acts in a disadvantageous manner as a relatively rigid counterforce. Thus, there is the danger that the pump piston may rebound and re-open the pressure line, and thereby cause unintended after-injections of fuel from the nozzle.
The known pump/nozzle is further provided with a relief channel arranged in the pump piston which furnishes a connection of the pressure line to a chamber of lower pressure, this connection being established at least substantially at the same time as the closing of the pressure line leading to the injection valve which is controlled by the frontal control edge at the end of delivery. By means of this connection, the pressure line is relieved to the level of the pressure in a return line, this level being provided by the pre-supply pump pressure. Accordingly, this action leads to a more rapid closing of the fuel injection valve. In a known pump/nozzle this relief action determines the end of fuel injection and only after this action is the connection to the pressure line blocked by the control edge on the frontal face of the pump piston and the pump piston is then braked. If this relief action is at the level of a very low standing pressure, in the vicinity of the supply pump pressure, then the pressure drop in the pressure line will take place more rapidly than the valve needle can close, and combustion gases can get into the nozzle openings of the injection valve, which can lead to their carbonization and to functional disturbances such that the injection valve may fail entirely. By means of the lowering of pressure in the pump work chamber before the pressure line is closed, the pressure in the adjacent filling chamber as well is lowered to the extent that no further usable damping effect on the end of the stroke motion of the pump piston is possible.
With pump/nozzles of similar design it is further possible to hinder the flow of fuel, which is forced out of the space between the servo piston and the pump piston before the end of pump delivery in such a manner that a motion damping occurs. However, since this motion damping takes place during the last measure of the delivery stroke, then the end of fuel injection is simultaneously postponed, which prevents the rapid needle closing required for modern engines. This postponement of the end of injection results in a corresponding worsening of the exhaust gas values, even without any consideration of a higher fuel consumption and unfavorable noise build-up.